Operational amplifiers (op amps) are often called upon to operate over a wide range of supply voltages. This can range from the typical 1.5 volts for a nearly discharged pair of carbon-zinc battery cells to many volts supplied from a plural cell battery. Efficient operation of such amplifiers commonly entails the use of a bootstrap output stage in which the output driver stage operating voltage is boosted my means of a feedback capacitor. This is done to enhance the drive to the output stage using circuitry that is well known in the art. At the higher operating supply voltages it is desirable for the d-c level of the output stage to operate at one half of the supply voltage. FIG. 1 of the drawing shows a commonly used biasing circuit. V.sub.s represents a supply voltage source connected between + terminal 10 and - terminal 11 which is grounded. Op amp 12 has an output terminal 13 which may be coupled to a loudspeaker or some other form of transducer (not shown). Inverting input 15 is directly coupled via resistor 16 to the output terminal 13 so that the amplifier has unity d-c gain. Resistor 17, which is bypassed to ground for signal frequencies by capacitor 18, reduces the a-c feedback and therefore determines the amplifier signal gain. Resistors 19 and 20 are typically of equal value and are coupled across V.sub.s so as to develop 1/2 V.sub.s at their juncture which is signal bypassed to ground by capacitor 22. Resistor 21 couples this voltage as a d-c bias to noninverting op amp input 14. This arrangement will make sure that the output operates at a d-c level equal to V.sub.s /2.
This circuit is commonly used but suffers a basic drawback when the bootstrap output stage is employed. For such an amplifier it has been found that below a critical supply voltage the V.sub.s /2 relationship is not desirable. For best overall performance below a critical or threshold supply voltage, the bias voltage should decrease more rapidly than V.sub.s /2. One commonly used method for achieving this is to include forwardly biased diodes in the bias circuit in a series parallel combination that causes the bias voltage to decrease faster than V.sub.s /2 below some threshold value. We have discovered that such diode biasing is limited to a threshold value of greater than 3 volts. We have further discovered that a threshold of just over 2 volts is desirable for many battery operated bootstrap amplifiers.